1. Field of the Invention
The present invention relates to a method for quantitative discharge of a molten material and, more particularly, to a method for quantitative discharge of a molten material from a melting furnace such as an electric furnace for the production of rock wool, devitro ceram and the like from molten blast furance slag.
2. Description of the Prior Art
As is known in the industry, molten slag available as by-product in large quantities from the blast furnace of a steel mill is quenched by water to produce water-granulated slag which finds its way into cement, blast furnace slag cement, soil improving agent, and aggregate for concrete. Also, molten slag is poured onto a slag bank and the air-cooled slag, after excavation, crushing and screening, is widely used as road base, aggregate for concrete, filling material for land reclamation, and raw material for rock wool.
Rock wool is conventionally made by mixing the crushed slag and supplementary materials added as needed to provide a specified raw material formulation and melting the mixture in a melting furnace such as an electric furnace and a cupola, charging the molten material to a fiberizing equipment and converting the material into fibers by the centrifugal force and/or a fluid such as compressed air and steam. The crushed slag thus used in the conventional method comes in a form convenient for transport and storage and also can be mixed readily with supplementary materials to give a specified raw material formulation for rock wool. On the other hand, the crushed slag causes a vast amount of sensible heat originally present in molten slag at the blast furnace site go to waste.
In recent years, from the standpoint of energy conservation, more attention is being paid to a process in which molten slag is charged into an electric furnace, the slag is adjusted for chemical composition and temperature there, and fed to a fiberizing equipment. This process requires only as much energy as to heat the molten slag slightly to melt away any supplementary material added to adjust the chemical composition, and realizes an exceptionally large reduction in energy requirement compared with the conventional method. A problem peculiar to this process, however, is that the blast furnace pours slag into a ladle intermittently at intervals of a few hours and the molten slag in the ladle which would radiate heat and solidify on standing needs to be charged into the electric furnace intermittently while still hot. As the commercial production of rock wool is normally a continuous operation, the molten material inside the electric furnace decreases continuously. For production of rock wool at a given level of quality in high yeilds, it is essential to minimize variations in the feed rate of the molten material to the fiberizing equipment, namely, to discharge the molten material from the electric furnace at a constant rate. This may be accomplished either by applying more pressure inside the electric furnace or by increasing the opening of the nozzle as the contents of the furnace decrease.
In practice, however, the former procedure will require separate installation of equipment for generation and control of pressure, resulting in a larger and more complicated system as a whole and further require to make the electric furnace an airtight pressure vessel. On the other hand, the latter will find a difficulty in adjusting the nozzle opening under precise control in response to the continuously changing weight of the contents of the furnace.